1. Field of the Invention
This invention relates to the field of optics and to an improvement of an interferometer useful for precise analytical determinations. More particularly, this invention relates in preferred forms to an interferometer incorporating an electrically alterable mask, a minicomputer and a detector which cooperatively facilitate the use of fast Fourier and Hadamard transforms or analogous mathematical multiplexing techniques to analyze electromagnetic radiation conforming to the laws of optics.
2. Description of the Prior Art
Interferometry has been known and used in the fields of physics and analytical chemistry for many years. A classical, simple interferometer consists basically of a light source, a light beam collimating device, beam splitter, two reflecting mirrors, a focusing lens, and a viewing surface. In operation, a beam of light from a light source strikes the beam splitter, for example a parallel faced optical flat with a thin detective coating one one surface, or Fresnel biprism, and is thereby divided into two separate, substantially identical beams of light. These two beams are reflected and directed by the reflecting mirrors such that the two beams become parallel to one another as they strike the focusing lens. The focusing lens is typically convex and causes the emerging beams to follow generally converging paths. The converging paths in turn cause the beams to electromagnetically interfere at an interference plane (this is the plane located at the focal point of the lens and perpendicular to the axis of the lens). For a visible light, the beams produce a bright region at the interference plane where they constructively interfere; the beams produce a darker region at the interference plane where they destructively interfere.
The pattern of interference observable on the interference plane is distinctive and characteristic of the electromagnetic radiation source and of any differences in refraction of the separate parallel beams. Because of this phenomenon, the interferometer is very useful, for example, in analyzing concentrations of gases dissolved in other gases or liquids, or for determining the spectral composition of a particular light beam. One difficulty with classical interferometry is that the interference pattern must be observed directly on the interference plane in real time. This makes long term, detailed analysis and measurement of the interference pattern difficult and tedious.
One improvement to classical interferometry in recent years is the development of techniques to produce an interferogram. An interferogram is a recording of an interference pattern from an interferometer which can be analyzed separately. An interferogram can be produced by placing photographic film at the interference plane. The film is exposed to the interference pattern, removed from the interferometer, developed, and then analyzed. While this is an improvement over classical interferometry, it is also time consuming and tedious.
The photographic film technique for producing interferograms can be used to analyze infrared radiation, but the same disadvantages of time-consuming and tedious analysis are present with infrared analysis as with visible light analysis. Additionally, the photographic technique is not readily adaptable to analysis by computer.
A very recent development in interferometry involves placing a solid state silicon photodiode array at the interference plane; see the article entitled "Fourier Transform Spectrometer With a Self-Scanning Photodiode Array" by T. Okamoto et al., at pages 269-273 of Applied Optics, Vol. 23, No. 2, Jan. 15, 1984. The photodiode array detects the bright regions in the interference pattern and converts these into electrical analog signals. The photodiode array is electrically coupled with a minicomputer which scans the array and converts the analog signals into digital signals for digital processing. The photodiode array permits quantitative detection of the interference pattern, and the microcomputer permits quick processing of the interference pattern data to mathematically construct an interferogram illustrative of the electromagnetic frequencies present and also their amplitudes. An interferogram so produced can be displayed on a cathode ray tube, X-Y plotter or stored on magnetic discs, magnetic tape, or the like.
The photodiode arrangement discussed above is useful for visible spectrum light, but presents problems when applied to infrared radiation. Analysis of infrared radiation presents special problems because known detectors are subject to extraneous infrared radiation which introduces "noise" into the infrared signal under study. Detectors such as cooled mercury-cadmium-telluride detectors have been developed to minimize the effect of unwanted infrared "noise." These detectors are expensive and not readily adaptable for use in a densely packed array.
A photodiode array similar to that discussed above but sensitive to infrared might be constructed but each photodiode would necessarily be a receptor of unwanted infrared "noise." The "noise" produced by such an array could be so great as to effectively obscure the infrared signal under analysis. If the number of photodiodes in the array is reduced, the noise is reduced proportionately, but then the resolution of the array is also reduced proportionately and again the detection of the infrared radiation under analysis is obscured.